In the market of medium to high quality colour hard copy derived from digital image information, thermal transfer printing is a leading technology, especially when using sublimation dye media. The main strengths of the technology lie in the reliability of the machines and their modest cost compared to photographic, electrophotographic or electrostatic printers. However, in terms of quality only the best and most expensive thermal printers approach photo-quality and for throughput and cost per copy all technologies trail behind electrophotography.
At present digital imaging of such materials is performed by thermal styli printheads. To provide a reasonable image quality, a high density of heat generating resistors is required which must be both accurately sized and of uniform resistance. To achieve this, several highly accurate microlithographic fabrication stages are required. The requirement that all the resistive elements must be functional and of uniform resistance at this level of fabrication complexity leads to a low yield and the cost of the thermal styli printhead is high. Applications requiring a higher resolution than 400 dots per inch (d.p.i.) are therefore not addressed by current thermal printing technology. Furthermore, only one of these high cost devices can economically be incorporated into a printing device. This causes serious disadvantages for full colour printing in which 3 or 4 colour separations must then be printed sequentially, which slows the throughput. Also the donor ribbon is printed in 3 or 4 sequential colours (cyan, magenta, yellow and optionally black) requiring the receptor paper to be re-registered for each colour on the single printhead. Each of the 3 or 4 colour ribbon donor sections is not normally re-usable and so the cost of a colour print is constant and high.
Therefore there is a need for a thermal imaging assembly having improved resolution and cost efficiency for multi-coloured printing.
IEEE Electron Device Letters, Vol. EDL3, P.254 (1982), Applied physics Letters, Vol. 42, p 484 (1983), U.S. Pat. Nos. 4,052,208 and 4,397,390, British Patent No. 2004077, French Patent No. 2402897, German Patent No. 2740835 and Japanese Patent No. 61010064 disclose thermoelectrographic processes having a thermal imaging device comprising a trilayer element of a photoconductor interposed between two electrodes, at least one electrode being substantially transparent. The second electrode or the photoconductor is thermally deformable or heat disintegrating such that following primary exposure to the image to be recorded and concomitant Joule heating arising from current flow in the conductive path, a permanent image comprising pits or holes is produced which is read for optical data storage.
U.S. Pat. No. 4,277,145, European Patent No. 12851 and German Patent No. 2904793 disclose an imaging assembly having a thermal imaging device of multilayer format in which a reflective second electrode is in intimate association with a liquid crystal layer. Isotropic change in the liquid crystal caused by Joule heating as the thermal imaging device is scanned and its subsequent cooling scatters light to produce an image for display.
German Patent No. 2904793 discloses an imaging assembly having a thermal imaging device comprising a support, an electrically conductive layer and a recording layer containing an oxidisable or reducible compound. A photoconductor with a conductive backing is brought into contact with the recording medium and upon light exposure, a current caused to flow through the recording medium produces a chemical reaction. The assembly is then heated at 130.degree. C. for 30 seconds to give a positive image of continuous tone.
U.S. Pat. No. 4,470,055 discloses a thermoelectrographic device having an ink transferral drum comprising a transparent substrate, a transparent electrode and a photoconductor. An ink, being solid at room temperature and having heat-fusing and semiconductive properties is coated onto the drum and paper brought into contact with the ink. With a voltage applied between the ink layer and the transparent electrode, illumination from within the drum causes the photoconductor to switch to a low resistance state and the Joule heating in the ink layer causes fusion and transferral of ink to the paper.
Japanese Patent No. 61244563 discloses a thermal imaging device comprising a transparent substrate, a transparent electrode, a photoconductor, a resistive heat generating layer and a further electrode. The device is addressed by a laser through the transparent substrate and electrode and in the light struck areas, the photoconductor switches to a low resistance state causing a large electric field to develop in the resistive layer. The Joule heating effect in this layer is then used to develop thermally sensitive paper.
The thermal image assembly incorporating such a thermal imaging device has a relatively low resolution (approximately 100 d.p.i.(4 dots per mm)).
A magneto-optic device is disclosed in the Journal of Applied Physics, Vol. 48 P.366 (1977), comprising non-magnetic garnet substrate bearing on one surface a ferrimagnetic garnet film, on which is deposited a first transparent electrode layer, followed by a photoconductor layer and a second transparent electrode layer. When a voltage is applied across the electrodes and the device is laser-exposed, sufficient joule heating occurs to bring about magneto-optical switching of the ferrimagnetic garnet layer. The device is useful for optical data storage.
Japanese Patent Application No. 63-159063 discloses a thermal imaging device comprising an amorphous silicon photoconductor sandwiched between two electrodes, at least one of which is transparent while the other bears a further wear-resistant coating. When a voltage is applied across the electrodes, and the device is illuminated by a laser diode through the transparent electrode, sufficient heat is generated via Joule heating to image thermally-sensitive paper held in contact with the wear-resistant layer. The performance quoted for this device includes a writing speed of 10.sup.-2 sec/mm, and a conductivity of 10.sup.-5 S/cm for illumination by a 5 mW laser diode emitting at 780 nm. Much higher writing speeds (e.g., by two orders of magnitude) are necessary for such a device to have practical applications. Much higher sensitivities are required, especially if more energy-demanding imaging media, such as dye-sublimation media, are to be employed.
U.S. patent application No. 4,638,372, European Patent Application No. 138221A, German Patent Application No. 3737449A1 and Japanese Patent Application No. 60085675A disclose thermal imaging assemblies having multiple printheads of the thermal stylus type for multi-colour thermal transfer printing but these devices have proven to be economically unfeasible.